This invention relates to an acid-corrosion resistant and wear resistant austenitic iron-base alloys that possess excellent resistance to sulfuric acid and are superior to high-speed steels and high-chromium, high-carbon type iron base alloys for many applications where both sulfuric acid corrosion and wear occur simultaneously. This invention further relates to such corrosion resistant alloys useful for making valve seat inserts used in internal combustion engines with an exhaust gas recirculation (EGR) system.
Internal combustion engines equipped with EGR systems require intake valve seat insert materials with excellent corrosion resistance due to the formation of sulfuric acid in the intake insert area when sulfur oxide that comes from diesel fuel after combustion meets with moisture from incoming air. Sulfur content in diesel fuel seems relatively low; however, the concentration of sulfuric acid will likely increase with engine running time as combustion deposits from exhaust gas accumulated around the inner wall area of an intake insert will absorb more sulfuric acid. Severe corrosion can occur on intake valve seat inserts made from M2 tool steel once the amount of high-concentration acid is enough. Cobalt-base alloy Stellite® 3 (Stellite is a Registered Trademark of Deloro Stellite Holdings Company) possesses excellent corrosion resistance and good wear resistance under diesel engine intake valve working conditions and therefore this cobalt alloy is normally the choice as the intake valve insert material to ensure the valve train service life in EGR device equipped diesel engines.
Traditionally, modified M2 tool steel and Silichrome XB are two common material choices for making diesel engine intake valve seat inserts. In broad ranges, modified M2 tool steel comprises 1.2-1.5 wt % carbon, 0.3-0.5 wt % silicon, 0.3-0.6 wt % manganese, 6.0-7.0 wt % molybdenum, 3.5-4.3 wt % chromium, 5.0-6.0 wt % tungsten, up to 1.0 wt % nickel, and the balance being iron. It is believed that Modified Silichrome XB contains 1.3-1.8 wt % carbon, 1.9-2.6 wt % silicon, 0.2-0.6 wt % manganese, 19.0-21.0 wt % chromium, 1.0-1.6 wt % nickel, and the balance being iron. Another common iron-base alloy for intake valve seat inserts contains 1.8-2.3 wt % carbon, 1.8-2.1 wt % silicon, 0.2-0.6 wt % manganese, 2.0-2.5 wt % molybdenum, 33.0-35.0 wt % chromium, up to 1.0 wt % nickel, and the balance being substantially iron. There are also several high chromium-type iron-base alloys available for making intake valve seat inserts.
U.S. Pat. No. 6,916,444 discloses an iron-base alloy containing a large amount of residual austenite for intake valve seat insert material. U.S. Pat. No. 6,436,338 discloses a corrosion resistant iron-base alloy for diesel engine valve seat insert applications. U.S. Pat. No. 6,866,816 discloses an austenitic type iron-base alloy with good corrosion resistance. However, more severe corrosion conditions in some engines with high sulfur fuel and high humidity demand materials with corrosion resistance much better than the above identified iron-base alloys.
High-carbon and high-chromium type nickel-base alloys normally do not exhibit good wear resistance under intake valve seat insert working conditions due to a lack of combustion deposits and an insufficient amount of metal oxides often found in exhaust valve applications, which help protect exhaust valve seat inserts from direct metal-to-metal wear. Eatonite® 2 (Eatonite is a Registered Trademark of Eaton Corporation) is one example of the nickel-base alloys used for making exhaust valve seat inserts, which is believed to contain 2.0-2.8 wt % carbon, up to 1.0 wt % silicon, 27.0-31.0 wt % chromium, 14.0-16.0 wt % tungsten, up to 8.0 wt % iron, and the balance being essentially nickel. Several similar nickel-base alloys with added iron and/or cobalt are also available for exhaust valve seat inserts. U.S. Pat. No. 6,200,688 discloses high-silicon and high-iron type nickel-base alloys used as material for valve seat inserts. These nickel-base alloys may possibly be used in EGR engines only when the wear rate of the intake valve insert is moderate.
Wear resistant cobalt-base alloys are another type of materials used in the industry, and the most commonly used ones are Stellite® 3 and Trilbaloy® T400 (Tribaloy is a Registered Trademark of Deloro Stellite Holdings Company) for more demanding applications. By way of background in U.S. Pat. Nos. 3,257,178 and 3,410,732, it is believed that Trilbaloy® T400 contains 2.0-2.6 wt % silicon, 7.5-8.5 wt % chromium, 26.5-29.5 wt % molybdenum, up to 0.08 wt % carbon, up to 1.50 wt % nickel, up to 1.5 wt % iron, and the balance being essentially cobalt. It is believed that Stellite® 3 contains 2.3-2.7 wt % carbon, 11.0-14.0 wt % tungsten, 29.0-32.0 wt % chromium, up to 3.0 wt % nickel, up to 3.0 wt % iron, and the balance being cobalt. The above cobalt-base alloys possess both excellent corrosion and wear resistance. However, the cost of these cobalt-base alloys only allows these alloys to be used in limited applications.
Austenitic iron-base valve alloys or valve facing alloys may also be classified into the same group of materials. U.S. Pat. No. 4,122,817 discloses an austenitic iron-base alloy with good wear resistance, PbO corrosion and oxidation resistance. U.S. Pat. No. 4,929,419 discloses a heat, corrosion and wear resistant austenitic steel for internal combustion exhaust valves. However, even in light of all of the above, there is still a need for a corrosion resistant iron-base alloy with good wear resistance, particularly an austenitic iron-base alloy with excellent corrosion resistance to meet the specific demand from more severe corrosion conditions in diesel engines with EGR systems.